Recent evidence indicates that cell interactions play a more extensive role than previously realized in early embryogenesis of the nematode C elegans, and that these interactions differ on the left and right sides of the embryo. Using these findings as a starting point, this project will investigate the genetic control of embryonic left-right asymmetry (handedness), the relative importance of cell-autonomous determinants vs. inductive events in determining the developmental fates of cells in the early embryo and their behavior during gastrulation, and the mechanisms of both cell-autonomous and inductive modes of determination. A combined genetic and molecular approach will be taken, including characterization and eventual cloning and analysis of mutationally defined genes, as well as cloning of genes directly as lineage-specific cDNA's followed by isolation of mutations in these genes to study their functions. Specifically, the following kinds of experiments will be undertaken. Mutations that cause reversal of embryonic handedness will be analyzed to define and characterize the genes and gene products involved in handedness choice. Cell contacts during early embryogenesis will be mapped by microscopy to define likely points of intercellular communication. Blastomere isolation and recombination experiments in culture as well as laser ablation experiments with whole embryos will be carried out to determine which of a set of lineage-specific differentiation markers are expressed cell-autonomously and which require specific cell-cell interactions. Mutations that cause defects in early inductive events and onset of gastrulation will be identified and analyzed, phenotypically in chimeric embryos to determine which specific cells they affect, as well as genetically to define the genes and gene products that control these processes. The polymerase chain reaction will be exploited to make blastomere-specific cDNA libraries from small numbers of dissected cells, which can be used in subtraction experiments to isolate cloned genes whose expression is specific to certain embryonic lineages. One of several possible "reverse genetic" methods will be used to obtain mutations in these genes for phenotypic analysis. Information on control of embryonic handedness could help to understand the causes of human situs inversus, which often is accompanied by heart defects and other malformations. Mechanisms of cell fate determination by either cell-autonomous determinants or cell interactions are clearly relevant to problems posed by cancer and inherited developmental disorders.